1. Field of the Invention
Engines or machines which produce mechanical output power, involving relative movement of two elements against a load drag or force, fall generally into two classes: translational, or linear, motors, and rotation motors. Where precise control or extremely long life are desired, electric motors are frequently preferred as the prime power source. However, because of many factors relating to the need for magnetic return paths and the cost of electromagnetic structures per unit volume, wherever a relatively long linear motion or stroke is desired starting with an electrical power source, the most common solution is to provide a rotary electric motor connected to a gear train driving a rack and pinion combination, or a rotating nut and threaded shaft combination. While multi-pole linear induction motors have been proposed as traction motors for electric railroads, these have not yet proved commercially practicable.
Many linear power sources are required to operate only over a finite distance, such as one-third to three times the principal dimension of a conventional electric motor producing the necessary amount of power. This is too long a stroke for efficient coupling of a single pole device such as a solenoid, so that in practice these applications frequently use a rotating motor which rotates the nut on a threaded shaft. However, where long life or sinusoidal linear movement of the output shaft are desired, this structure is not a very satisfactory solution.
2. Description of the Prior Art
It was recognized long ago that the basic design of a polyphase synchronous motor could be followed, to provide a linear motor having a linearly extended series of transverse pole structures. A 3-phase structure of this type is described in the paper "Commutation and Control of Step Motors" by Ish-Shalom and Manzer, Proceedings of the 14th Annual Symposium, Incremental Motion Control Systems and Devices, published by the Incremental Motion Control Systems Society in June, 1985. While published after the conception of the invention embodied in FIGS. 1-3 herein, this paper provides a useful background description of the electronic drive, and the principles underlying such a linear motor.
To achieve the high-power density associated with the use of a permanent magnet as a field magnetism source, the so-called "hybrid stepping motor" was developed and is described, for example, in a published thesis, "Static Performance of a Hybrid Stepping Motor with Ring Coils", by Ben. H. A. Goddijn, Sept. 9, 1980, published in Waalre, The Netherlands. In such a hybrid motor a field flux passes through a total motor magnetic circuit, arranged with teeth extending transversely to the direction of linear motor, the teeth on the stator having the same pitch as those on the rotor, but with certain teeth offset by 90.degree. of one full cycle of electrical excitation in what is, effectively, a two phase electric motor. Each of the two driving coils affects half of the teeth on the armature, adding to the flux in one group of teeth to raise the flux level nearly to saturation, and bucking the field flux in the other teeth down to nearly zero. Relative movement occurs so that a tooth on the field structure is brought into alignment with the tooth which has been driven nearly to saturation.
Such a motor, as a linear structure, is clearly the linear equivalent of the rotary synchronous or stepping motor disclosed in U.S. Pat. No. 4,206,374. While it is possible to design such a structure for linear operation, the mass of the displacer or linearly moving element will be relatively high, and the manufacturing cost will also be quite high because of the large numbers of transversely extending teeth which are required. If the displacer is made shorter than the stator, to reduce the moving mass, and the coils are placed on the stator for the same reason, then a great number of coils are required to produce the necessary alternation of polarities along the length of the stator. This makes such a motor extremely expensive, and also requires that the poles be relatively far apart in the longitudinal direction. If such a motor is stepped by applying full current alternatively to one coil or the other, relative coarse stepping motion is obtained.